The "What is a moon?" definition thread

[Ara Pacis]

OK, here it is. What's a moon? Is there only one moon, The Moon. Should the question be "What is a Natural Satellite? Should it be defined based on size, composition, or origin, longevity, parent object?

How big does a moon need to be? Large enough to be detectable is a start, but is it a definition? If there is a size limit, where should it be set?

Is composition important? Can it be its own life bearing world and still be a moon or should it be limited to airless worlds? Should it be similar or dissimilar to the parent planet.

Is the genesis, or origin, of the natural satellite important? Does co-formation make it a moon or a binary planet? Should a moon come from the planet originally or can it be captured? Is capture the only way to get a moon and any other genesis is something else entirely?

Should it be based on longevity? The earth's biggest natural satellite, sometimes called Luna, has been orbiting the earth for a period less than the earth's existence. On the other hand, scientists have detected several small asteroids that orbit the earth in corkscrew orbits for years at a time - Are those objects natural satellites? Are they moons? Are they both?

Should it be based on what it is orbiting? A planet is a natural satellite of a star or stellar remnant. What about natural satellites orbiting Almost Stars, popularly known as brown dwarfs? Some objects orbit the star as primary with periods or influence by a larger mass. Is it a moon if it is co-orbital or does it need to be beholden to the larger mass? Does a moon fall in towards the parent planet or does it move away?

I'm not expressing any preference, yet. What's your definition?

[Tinaa]

Moved to questions and answers

[Jeff Root]

Anything that is orbiting a planet.

-- Jeff, in Minneapolis

[Saluki]

Anything that is orbiting a planet.

-- Jeff, in Minneapolis

Including discarded rocket stages? What about dust specs? Are the rings of Saturn really millions of moons?

[Jeff Root]

Anything that is orbiting a planet.

Including discarded rocket stages?

Sure.

What about dust specs?

I guess so. I can't think of any reason not to.

Are the rings of Saturn really millions of moons?

What do you mean by "really"?

Assuming the word "millions" was arbitrarily chosen to
represent the uncountably vast number of objects comprising
Saturn's rings, if you had asked, "Are the rings of Saturn
composed of an uncountably vast number of moons?" I would
answer, "Yes".

-- Jeff, in Minneapolis

[Saluki]

How about human waste discharged from the shuttle, ISS and other manned vessels?

[tdvance]

I'm not sure what the official definition of a moon is, but I would propose: a body held together by its own gravity in orbit around a non-stellar body.

Under that definition, the earth has three moons--the one we all know, and two "temporary" moons....captured asteroids that are not in long-term stable orbits around the Earth.

See http://science.nasa.gov/headlines/y2...htm?list185042

Todd

[Saluki]

I would argue that the definition should follow these rules:

1. Man-made objects are excluded. If we put it there, it is an artificial satellite, not a Moon.

2. Only objects with orbits that are stable for a reasonably long term (actual length of time is debatable, but many times longer than a Human life) are included.

3. Above a certain size. The size cut-off is debatable, but I would certainly not include anything so small that it is not held together by its own gravity. When I think of a moon, I think of something large enough that a small group of people could land on it in a space ship, and not worry about significantly altering the moon's orbit.

[korjik]

A natural object held together by its own gravity that orbits a planet.

[Saluki]

I just thought of #4.

4. Objects orbiting in similar orbits with many other similar objects are excluded from the definition of the moon. If Ceres is not a planet, then a big ring particle is not a moon.

[Ara Pacis]

I would argue that the definition should follow these rules:

1. Man-made objects are excluded. If we put it there, it is an artificial satellite, not a Moon.

2. Only objects with orbits that are stable for a reasonably long term (actual length of time is debatable, but many times longer than a Human life) are included.

3. Above a certain size. The size cut-off is debatable, but I would certainly not include anything so small that it is not held together by its own gravity. When I think of a moon, I think of something large enough that a small group of people could land on it in a space ship, and not worry about significantly altering the moon's orbit.

very anthropocentric

[tdvance]

I think i'd better add the requirement that both the moon and planet be substellar. Otherwise, one could consider the Magellanic clouds to be moons.

[Saluki]

very anthropocentric

What is your point? All of scientific jargon is arbitrarily selected for the convienience of the organism using the word. If it were a dog defining the term, it would probably relate to the suitability of the object for leg heisting.

[tony873004]

I'm not sure what the official definition of a moon is, but I would propose: a body held together by its own gravity in orbit around a non-stellar body.

Under that definition, the earth has three moons--the one we all know, and two "temporary" moons....captured asteroids that are not in long-term stable orbits around the Earth.

See http://science.nasa.gov/headlines/y2...htm?list185042

Todd

Those temporary "moons" shouldn't be counted at all. They "orbit" Earth in the sense that they travel around Earth in a rotating frame of reference, but they are not gravitationally bound to Earth. If the Sun suddenly disappeared, Earth would retain the Moon as it drifted through interstellar space. The temporary "moons" would immediately depart on their own seperate trajectories throug interstellar space.

[Jeff Root]

I'm not sure what the official definition of a moon is

There is no "official" definition.

I would propose: a body held together by its own gravity in
orbit around a non-stellar body.

Under that definition, the earth has three moons--the one we
all know, and two "temporary" moons....captured asteroids that
are not in long-term stable orbits around the Earth.

I agree with Tony. The other two bodies you refer to are not
"captured" and are not orbiting Earth. Thus, not moons.

In another thread (I forget which) in the last couple of days,
someone said that the Moon does not orbit the Earth. (Meaning
that it orbits the Sun but not the Earth.) That, too, is wrong.
The Moon unambiguously orbits the Earth.

-- Jeff, in Minneapolis

[Jeff Root]

I would argue that the definition should follow these rules:

1. Man-made objects are excluded. If we put it there, it is an
artificial satellite, not a Moon.

What was the first use of the term "satellite" in astronomy?
The use which established the astronomical meaning of the term,
and which it retains to this day?

"Satellite" is the term Galileo applied to the four objects
he saw orbiting Jupiter: The moons Io, Europa, Ganymede, and
Callisto. Moons. Satellite = Moon.

Luna is the only known natural satellite of Earth. Thousands
of small, artificial moons are currently orbiting the Earth.

2. Only objects with orbits that are stable for a reasonably
long term (actual length of time is debatable, but many times
longer than a Human life) are included.

Why?

3. Above a certain size. The size cut-off is debatable, but I
would certainly not include anything so small that it is not
held together by its own gravity.

Why?

When I think of a moon, I think of something large enough that
a small group of people could land on it in a space ship, and
not worry about significantly altering the moon's orbit.

When I think of a moon, I think of something orbiting a planet.

-- Jeff, in Minneapolis

[Saluki]

What was the first use of the term "satellite" in astronomy?
The use which established the astronomical meaning of the term,
and which it retains to this day?

"Satellite" is the term Galileo applied to the four objects
he saw orbiting Jupiter: The moons Io, Europa, Ganymede, and
Callisto. Moons. Satellite = Moon.

Absolutely correct, but moon ≠ artificial satellite. At least not in any scientific usage I have ever seen.

Luna is the only known natural satellite of Earth. Thousands
of small, artificial moons are currently orbiting the Earth.

I suppose I can allow that with the word "artificial" inserted. Again, moon ≠ artificial satellite, but perhaps artificial moon = artificial satellite.

Why?

Why not? Word definitions are based on usage. An object that temporarily orbits, and then flys off somewhere else is not typically called a moon.

Why?

Again, why not? We do not commonly call random space debris moons. Look at rings again as an example. The small moons responsible for the gaps are called moons ("sheppard moons", in fact), but the actual particles that make up the rings are not.

When I think of a moon, I think of something orbiting a planet.

-- Jeff, in Minneapolis[/quote]

Fair enough.

[Kaptain K]

The Moon unambiguously orbits the Earth.

Only from a geocentric point of view. It would be more accurate to say that the Earth and the Moon share an orbit around the Sun. The Moon's path is always concave to the Sun, as is the Earth's. This is not true of any other moon in the solar system!

[Jeff Root]

2. Only objects with orbits that are stable for a reasonably
long term (actual length of time is debatable, but many times
longer than a Human life) are included.

Why?

Why not?

But why?

Word definitions are based on usage. An object that temporarily
orbits, and then flys off somewhere else is not typically called
a moon.

No? What is it typically called?

What precedent of usage are you trying to adhere to?

3. Above a certain size. The size cut-off is debatable, but I
would certainly not include anything so small that it is not
held together by its own gravity.

Why?

Again, why not?

And again, why?

We do not commonly call random space debris moons.

Why not?

I don't know what "random space debris" is. What distinguishes
it from non-random space debris? What distinguishes it from
random space non-debris?

Do we commonly call random space debris (whatever it is) anything?
If so, what do we call it?

Look at rings again as an example. The small moons responsible
for the gaps are called moons ("sheppard moons", in fact), but
the actual particles that make up the rings are not.

I called them moons 14 posts back in this very thread.
Obviously they are called "moons" at least occasionally.

The correct spelling is "shepherd".

-- Jeff, in Minneapolis

[Halcyon Dayz]

Only from a geocentric point of view. It would be more accurate to say that the Earth and the Moon share an orbit around the Sun. The Moon's path is always concave to the Sun, as is the Earth's. This is not true of any other moon in the solar system!

Maybe that is because the Moon is the moon closest to the Sun.

Satellite: "A satellite is any object that orbits another object" and "The general criterion for an object to be a satellite is that the center of mass of the two objects is inside the primary object."

The Moon is Earth's natural satellite.
According to Wiki: "A natural satellite is a non-man-made object that orbits a planet. It is commonly referred to as a moon (not capitalized)."
It also includes those orbiting minor planets.

But, if only for purely practical reasons, we can not include every dust and ice particle that happens to orbit a planet. Those would be satellites but not moons.

For me a moon is something I could walk on.

[Saluki]

I called them moons 14 posts back in this very thread.
Obviously they are called "moons" at least occasionally.

Ahh, we are now at the root of the matter. They are moons because Jeff calls them moons. I see now. Thank you for clarifying.

[jseefcoot]

If you take what people are repeating the most, the definition of a moon could be: A naturally occuring celestial body, large enough to be held together by its own gravity, that is gravitationally bound to another in a stable orbit.

But I wonder if a distinction should be made between our Moon (or Luna) and other satellites. . . .similar to the debate over whether to call Pluto a planet or not. How important is such a specific definition when our knowledge base of space and astronomy outgrows itself constantly? Often, as soon as we define something, we learn something new that challenges that definition. To me it's pure semantics, and in a country where a house can burn up while it burns down, and where maters, tuhmatuhs and tomatoes are all the same thing, it doesn't make much difference how we define it, as long as we all know what we're talking about.

[Jeff Root]

The Moon unambiguously orbits the Earth.

Only from a geocentric point of view.

The "point of view" is irrelevant.

It would be more accurate to say that the Earth and the Moon
share an orbit around the Sun.

It is accurate to say that, but not more accurate.

The Earth orbits the Sun. The Moon orbits the Sun. The Moon
orbits the Earth. The Earth-Moon system orbits the Sun.

With somewhet less clarity, we could also say:

The Sun orbits the Earth-Moon system. The Sun orbits the Earth.
The Earth orbits the Moon. The Sun orbits the Moon.

Although those orbits are far smaller than the first four.

The Moon's path is always concave to the Sun, as is the Earth's.
This is not true of any other moon in the solar system!

That is an interesting bit of trivia, but it doesn't have any
impact on the fact that the Moon orbits the Earth. If the Sun
were to instantaneously blink out of existence, although the
variation over time of the Moon's orbit around the Earth would
be altered, the orbit itself would not. The sidereal period
would be the same, the apogee and perigee would be the same.

You read or heard somewhere that the Moon is in orbit around
the Sun, not the Earth. Whoever wrote or said that was wrong.

-- Jeff, in Minneapolis

[Ara Pacis]

Some people want the term "moon" (non-capitalized) to mean a natural satellite. Some further assign or assume mass or size limits for that term. Some people think that Moon (capitalized) is the proper name of Earth's neighbor. I'd be willing to call earth's moon Luna and leave the word moon as a generic noun if we can keep using geology/geography, perigee/apogee, and other geo-derived words generically. As we explore new worlds I don't want to have to memorize some strange new science discipline names like areology, selenography, and perijove.

Also, did y'all see the post in Astronomy about a new theory for moon formation? The moons of Jupiter and Saturn suggest a standard formation limit of .01% of the parent planet's mass. Triton is over this limit is thought to be captured instead. Luna is 1% of Earth's mass, roughly 100 times more massive than the formation limit calls for. Should this datum support the issue that Luna is not a moon but a binary planet with Earth?

[tony873004]

...The Earth orbits the Sun. The Moon orbits the Sun. The Moon orbits the Earth...

Interesting trivia: Although this is true for Earth's moon, this isn't true for a lot of the moons of the Solar System.

Jupiter's moon, Io spends about 3/4 of its orbit around Jupiter with a velocity relative to the Sun that exceeds solar escape velocity for that distance. For most parts of its orbit, if Jupiter suddenly disappeared, it would leave Io on a hyperbolic solar trajectory.

But when Io is inbetween the Sun and Jupiter, and for ~1/4 of its orbit centered around that point, Io's velocity relative to the Sun is below solar escape velocity. If Jupiter disappeared during this quarter of Io's orbit, Io would remain bound to the Sun.

One could argue that Jupiter is constantly ejecting and capturing Io from the Solar System.

Could one make the argument that sometimes Io is not part of our solar system as its solar trajectory is hyperbolic?

The same holds true for the other 3 Galilean moons, and a lot of the other moons of the outer planets, but as distance from the planet increases, they spend a larger percent of their orbit below solar escape velocity.

[Halcyon Dayz]

Some people think that Moon (capitalized) is the proper name of Earth's neighbor.

The proper name of Earth's neighbour is Maan.
The rest of the world is just being stubborn.
Luna is just something the Romans came up with.

Luna is 1% of Earth's mass, roughly 100 times more massive than the formation limit calls for. Should this datum support the issue that Luna is not a moon but a binary planet with Earth?

Since it originated the word, the Moon is by definition a moon.
The datum does support the notion that our Moon was formed differently then other moons though.

ADD: If it turns out that the Earth and Moon are binary planets, then the term moon
should be reserved for the smaller element of such a system.
Natural satellites should then be called just that.

[Van Rijn]

The proper name of Earth's neighbour is Maan.
The rest of the world is just being stubborn.
Luna is just something the Romans came up with.

Sounds good to me, if it will stop this silly bickering.

Though I prefer Selene . . .


For a "moon," I'd go with: Natural satellite of a planet, with some minimum size, not necessarily held together significantly by gravity (at the lower end of the scale it really doesn't take much of an impact to knock bits off, and if there is rotation . . .), but structurally stable. We're starting to run into the same problems with "moons" or satellites as we are with KBOs - as the detection capabilities grow, the number of "moons" keep increasing. At some point you have to decide on the dividing line between orbital debris and natural satellites.

[Jeff Root]

But, if only for purely practical reasons, we can not include
every dust and ice particle that happens to orbit a planet.

What practical reasons? I'm not aware of any practical reason
not to call every dust and ice particle that happens to orbit
a planet "a moon".

Those would be satellites but not moons.

Why? How is a satellite different from a moon? To me they are
synonymous terms. They became synonymous when it was understood
that the satellites of Jupiter are analogous to Earth's Moon, and
the term "moons" was applied to them by English-speaking people.

Some 350 years later, the terms "moon" and "satellite" were both
applied to the objects put into orbit by the Soviet Union and the
United States. I don't recall anyone objecting to that use of
the terms at the time. (Okay-- I probably wouldn't have known
if there were any objections, because I was just a little kid.)

-- Jeff, in Minneapolis

[Jeff Root]

Some people want the term "moon" (non-capitalized) to mean a
natural satellite.

I would go along with that as the preferred meaning, but there
is significant historical precedent (perhaps beginning with
the 1869 story "The Brick Moon" by Edward Everett Hale) for
applying the term "moon" to artificial satellites, as well.
Sputnik, Vanguard, and Explorer were frequently described as
"artificial moons". But I can accept that that usage is no
longer current.

Some further assign or assume mass or size limits for that term.

Without explaining why.

Some people think that Moon (capitalized) is the proper name of
Earth's neighbor.

People whose first language is English, yes.

Also, did y'all see the post in Astronomy about a new theory for
moon formation? The moons of Jupiter and Saturn suggest a standard
formation limit of .01% of the parent planet's mass. Triton is
over this limit is thought to be captured instead. Luna is 1% of
Earth's mass, roughly 100 times more massive than the formation
limit calls for. Should this datum support the issue that Luna is
not a moon but a binary planet with Earth?

I'd call that a total non sequitur. I have no problem with
referring to the Moon as a planet, but since it orbits another
planet, it is a moon. That's completely unambiguous. You can
call the Earth-Moon system a "binary planet" if you like (I'm
not sure what it means, but it does seem descriptive), but use
of that term has no bearing on the fact that the Moon is a moon.

-- Jeff, in Minneapolis

[Jeff Root]

One could argue that Jupiter is constantly ejecting and capturing
Io from the Solar System.

Could one make the argument that sometimes Io is not part of our
solar system as its solar trajectory is hyperbolic?

But it isn't. Jupiter is part of the Solar System, and would
have to magically vanish in order for Io to be on a hyperbolic
escape trajectory from the Solar System. And since Jupiter
is what causes Io to have the high speed that it has, if
Jupiter disappeared, Io could not be on a hyperbolic escape
trajectory at all. Nomatter how you analyze it, Io is
gravitationally bound to the Solar System.

-- Jeff, in Minneapolis